In electrophotography photoconductive materials are used to form a latent electrostatic charge image that is developable with finely divided colouring material, called toner.
The developed image can then be permanently affixed to the photoconductive recording material, e.g. photoconductive zinc oxide-binder layer, or transferred from the photoconductor layer, e.g. selenium layer, onto a receptor material, e.g. plain paper and fixed thereon. In electrophotographic copying and printing systems with toner transfer to a receptor material the photoconductive recording material is reusable. In order to permit a rapid multiple printing or copying a photoconductor layer has to be used that rapidly looses its charge on photo-exposure and also rapidly regains its insulating state after the exposure to receive again a sufficiently high electrostatic charge for a next image formation. The failure of a material to return completely to its relatively insulating state prior to succeeding charging/imaging steps is commonly known in the art as "fatigue".
The fatigue phenomenon has been used as a guide in the selection of commercially useful photoconductive materials, since the fatigue of the photoconductive layer limits the copying rates achievable.
Another Important property which determines whether or not a particular photoconductive material is suited for electrophotographic copying is its photosensitivity that must be high enough for use in copying apparatus operating with a copying light source of fairly low intensity.
Commercial usefulness further requires that the photoconductive layer has a chromatic sensitivity that matches the wavelength(s) of the light of the light source of the copier, e.g. laser, or has panchromatic sensitivity to allow the reproduction of all colours in balance.
Intensive efforts have been made to satisfy said requirements, e.g. the spectral sensitivity of selenium has been extended to the longer wavelengths of the visible spectrum by making alloys of selenium, tellurium and arsenic. In fact selenium-based photoconductors remained for a long time the only really useful photoconductors although many organic photoconductors were discovered.
Organic photoconductor layers of which poly(N-vinylcarbazole) layers have been the most useful were less interesting because of lack of speed, insufficient spectral sensitivity and rather large fatigue.
However, the discovery that 2,4,7-trinitro-9-fluorenone (TNF) in poly(N-vinylcarbazole) (PVCz) formed a charge-transfer complex strongly improving the photosensitivity (ref. U.S. Pat. No. 3,484,237) has opened the way for the use of organic photoconductors in copying machines that could compete with the selenium-based machines.
TNF acts as an electron acceptor whereas PVCz serves as electron donor. Films consisting of said charge transfer complex with TNF:PVCz in 1:1 molar ratio are dark brown, nearly black and exhibit high charge acceptance and low dark decay rates. Overall photosensitivity is comparable to that of amorphous selenium (ref. Schaffert, R. M. IBM J. Res. Develop., 15. 75 (1971).
A further search led to the discovery of phthalocyanine-binder layers, using poly(N-vinylcarbazole) as the binder [ref. Hackett, C. F., J. Chem. Phys., 55, 3178 (1971)]. The phthalocyanine was used in the metal-free X form and according to one embodiment applied in a multilayer structure wherein a thin layer of said phthalocyanine was overcoated with a PVCz layer. Hackett found that photoconductivity was due to field dependent photogeneration of electron-hole pairs in the phthalocyanine and hole injection into the PVCz. The transport of the hole carriers, i.e. positive hole conduction proceeded easily in the PVCz layer. From that time on much research has been devoted to developing improved photoconductive systems wherein charge generation and charge transport materials are separate in two contiguous layers (see e.g. U.K. Pat No. 1,577,859). The charge generating layer may be applied underneath or on top of the charge transport layer. For practical reasons, such as less sensitivity to wear and ease of manufacture, the first mentioned arrangement is preferred wherein the charge generating layer is sandwiched between a conductive support and a light transparent charge transport layer as described e.g. by Wolfgang Wiedemann In the article titled: "Organische Photoleiter--Ein Uberblick, II" published in Chemiker Zeitung, 106. (1982) Nr. 9 p. 315.
Photoconductive recording materials as can be learned from the preamble may exist in different configurations with one or more "active" layers coated on an electrically conductive substrate. By active layer is meant a layer that plays a role in the formation of the electrostatic charge image. Such layer may be a monolayer photoconductive layer being responsible for both charge carrier generation and charge carrier transport or in a system of two active layers a layer that is responsible for charge carrier generation or charge transport.
In order to form a photoconductive two layer-system with high photosensitivity to the incident light efficient charge generating substances are required that operate in conjunction with efficient charge transport substances.
Various organic dyes and pigments have been proposed as charge generating substances.
Most of the organic dyes or pigments of the charge generating layer provide more efficient hole injection than electron injection across a field-lowered barrier at the interface where pigment-dye/charge transport compounds touch each other and a charge transfer complex compound is possibly formed.
Efficient p-type charge transport compounds can be found in the group consisting of aromatic homocyclic amino compounds, e.g. triarylamino compounds, heteroaromatic amino compounds, unsaturated heterocyclic amino compounds, hydrazone compounds, triphenylmethane, oxazole and pyrazoline derivatives or polymeric p-type charge transport substances incorporating groups corresponding with one of the above types of compounds.
Examples of double layer systems containing heteroaromatic compounds or amino-substituted unsaturated heterocyclic compounds as charge transporting substances are described in DE-P 2 237 539, U.S. Pat. No. 3,837,851 and in our co-pending published European Patent applications (EP-A) 347 960, 347 967 and 349 034 and in unpublished European Patent Applications Nos. 89200707.1 and 90201600.5.
Examples of triarylamino derivatives that are useful as charge transporting compounds in a double layer photoconductive system are given in U.S. Pat. No. 4,265,990.
Examples of hydrazone compounds as charge transporting substances are described in U.S. Pat. Nos. 4,278,747 and 4,365,014 and in our co-pending unpublished European Patent Applications Nos. 90200717.8 and 90200968.7.
Examples of triphenylmethane derivatives that are useful as charge transporting compounds in a double layer photoconductive system are given in U.S. Pat. Nos. 4,050,935, 4,140,529 and 4,330,608.
Efficient n-type charge transport compounds can be found in the group consisting of aromatic ketones optionally substituted with at least one electron withdrawing substituent, e.g. halogen, nitro, nitrile, carbamate group, acyl group or carboxylic acid ester group and optionally condensed with malonodinitrile, a malononitrile monocarboxyester or a malonic acid diester; cyanoalkylene compounds, aromatic compounds with electron withdrawing substituents, aromatic ketones in which one or more of the keto-groups have been substituted by two mercapto groups, a sulphone group, an imino cyanide group (.dbd.N--CN) or polymeric n-type substances incorporating groups corresponding with one or more of the above mentioned types of compounds.
Examples of aromatic ketones substituted with at least one electron withdrawing substituent are described e.g. by R. O. Loutfy, C. K. Hsiao, B. S. Ong and B. Keoshkerian In Canadian Journal of Chemise, Vol. 62, p. 1877 (1984). These authors also describe aromatic ketones condensed with malonodinitrile, malononitrile monocarboxyester or a malonic acid diester and aromatic ketones in which keto groups are replaced by two mercapto groups. Aromatic ketones in which at least one of the keto groups has been replaced by an imino cyanide group or a sulphone group are described In DE-OS 3 437 814 and U.S. Pat. No. 4,514,481 respectively.
Useful charge generating substances are In the following classes:
a) perylimides, e.g. C.I. 71 130 (C.I.=Colour Index) described in DBP 2 237 539, PA1 b) polynuclear quinones, e.g. anthanthrones such as C.I. 59 300 described in DBP 2 237 678, PA1 c) quinacridones, e.g. C.I. 46 500 described in DBP 2 237 679, PA1 d) naphthalene 1,4,5,8-tetracarboxylic acid derived pigments including the perinones, e.g. Orange GR, C.I. 71 105 described in DBP 2 239 923, PA1 e) phthalocyanines, e.g. H2-phthalocyanine in X-crystal form (X--H.sub.2 Pc), metal phthalocyanines, e.g. copper phthalocyanine, C.I. 74 160 described in DBP 2 239 924, indium phthalocyanine and titanyl phthalocyanine described in U.S. Pat. Nos. 4,713,312 and 4,728,592, PA1 f) indigo- and thioindigo dyes, e.g. Pigment Red 88, C.I. 73 312 described in DBP 2 237 680, PA1 g) benzothioxanthene-derivatives as described e.g. in DAS 2 355 075, PA1 h) perylene 3,4,9,10-tetracarboxylic acid derived pigments including condensation products with o-diamines as described e.g. in DAS 2 314 051, PA1 i) Polyazo dyes or pigments, e.g. Chlordiane Blue C.I. 21 180 described in DAS 2 635 887, the bisazo pigments described in DOS 2 919 791, DOS 3 026 653, DOS 3 032 117 and U.S. Pat. No. 4,123,270, and particular trisazo-pigments described in U.S. Pat. No. 4,687,721. PA1 j) squarilium dyes as described e.g. in DAS 2 401 220, PA1 k) polymethine dyes. PA1 l) triarylmethane dyes, and PA1 m) dyes containing 1,5 diamino-anthraquinone groups. PA1 A Is nitrogen or CR, with R being hydrogen, alkyl with 1 to 4 carbon atoms, phenyl, which may be substituted by alkyl with 1 to 4 carbon atoms or halogen, and PA1 B is an m-phenylene, 2,6-pyridylene, 1,3-isoindylene or pyrrolylene group, and PA1 D is hydrogen, or two groups D together stand for a group ##STR3## with the proviso that 1. if B is m-phenylene or 2,6-pyridylene, A is nitrogen and two groups D together stand for a group ##STR4## 2. if B is 1,3-isoindolylene, A is nitrogen and two groups D together stand for a group ##STR5## 3. if B is pyrrolylene, A is CR and D is hydrogen.
Charge generating dyes and pigments not only have to fulfill the requirement of a high charge generating capacity in the spectral range of practically useful exposure sources such as incandescent light bulbs, fluorescent light tubes and lasers, but have to maintain that property for repetitive use without unacceptable deterioration in characteristics
A search is still going on to find charge generating substances that optimally combine light-sensitivity without deterioration with high charge generating capacity and low residual potential after exposure. With the advent of compact near IR-emitting laser diodes with emission at 780 to 850 nm depending on the laser diode composition, there has been increasing demand for charge generating pigments with maximum sensitivity in this wavelength range.
U.S. Pat. No. 3,895,944 discloses a charge carrier producing dyestuff layer of a dyestuff corresponding to the general formula: ##STR2## wherein: Z is hydrogen or two groups Z together stand for a bivalent metal atom,
Said patent further discloses that when used In multilayer materials, with a top charge transporting layer, the phthalocyanines display a very high degree of photosensitivity within the spectral range between 550 and 750 nm. Furthermore it is evident from the examples that multi-layer photoconductors with porphyrin pigments such as meso-tetraphenylporphyrin exhibit vastly inferior photosensitivities to those with phthalocyanine pigments.
Surprisingly it has been found that certain porphyrin pigments exhibit superior photosensitivities when incorporated in multilayer or single layer organic photosensitive materials and furthermore that particular porphyrin pigments exhibit high photosensitivities in the wavelength range 780 to 850 nm with maximum sensitivities in said wavelength range.